The present invention relates to a reactor core arrangement and, more particularly, to an improvement in the fuel assembly arrangement in the reactor core of a boiling water reactor.
In the conventional boiling water reactor (BWR), the control of excess reactivity, as well as the control of distribution of power, is made by changing the insertion degree of control rods, the flow rate of the coolant and the amount of burnable poison contained in the fissionable material.
A control rod operation for changing the insertion degree of the control rods is liable to incur a drastic change in the power peaking which in turn causes a breakdown of the fuel rods. Therefore, it is current practice when the control rods are operated to previously lower the power of the reactor down to 20%, or so, of the rated power, by adjusting the flow rate of the coolant flowing through the reactor core, in order to avoid drastic changes of power peaking. Thus, lowering of the reactor output is necessary at each time of operation of the control rod. For this reason, the plant capacity factor of the nuclear reactor is considerably lowered, particularly when the frequency of control rod operation is high.
It is, therefore, a better strategy to eliminate the control rod operation for control of excess reactivity and power distribution.
Under these circumstances an acceptable measure is to control excess reactivity by adjusting the amount of burnable poison, which is a neutron absorbent, contained in the fuel in the reactor core. Namely, it is possible, by a use of the burnable poison, to minimize the requirement of mechanical control rod operation, thereby to increase the plant capacity factor of a nuclear reactor. Further, it is possible to control power distribution in the core by suitably distributing a burnable poison in the reactor core.
The specification of U.S. Pat. Application Ser. No. 936,133 discloses a fuel assembly containing burnable poison, having a plurality of fuel rods containing burnable poison.
This fuel assembly is constituted by a plurality of groups of fuel rods, some of the groups having fuel rods containing a burnable poison. According to this U.S. Patent Application, the groups of fuel assemblies placed within the 50% central area of the reactor core are regarded as being constituted by a plurality of units, each unit including four fuel assemblies. Each time there is a replacement of fuel, one of four fuel assemblies of each unit is replaced with a fuel assembly containing a burnable poison, thereby permitting easy control of excess reactivity, as well as power distribution, in a minimized control rod operation. The method disclosed in the above-mentioned specification also makes it possible to minimize the troublesome shuffling of the fuel assemblies required in the fuel replacement.
A similar fuel assembly containing burnable poison is disclosed also in the specification of the U.S. Pat. Application Ser. No. 942,555. In this fuel assembly, the ratio of the poison-containing fuel rods to all fuel rods of each fuel assembly is so selected that the burnable poison in the fuel assembly is exhausted when one cycle of operation of the nuclear reactor is completed.
The specification of U.S. Pat. Application Ser. No. 952,660 discloses the use of a fuel assembly containing a poison for obtaining a constant excess reactivity in accordance with an increase in the degree of fuel burn-up (exposure). This patent application shows that four fuel assemblies of different burning histories in combination constitute a unit, and of which one fuel assembly having the oldest burning history is replaced with a fuel assembly containing a poison each time one operation cycle of the nuclear reactor is completed. In U.S.P. Appln. Ser. No. 952,660, it is also disclosed to flatten the axial power distribution in the reactor core by providing a higher fuel enrichment in the upper part than in the lower part of the reactor core.
The above mentioned United States Patent Applications propose the use of fuel assemblies containing a burnable poison, in order to minimize the frequency of the mechanical control rod operation. Particularly, the specification of U.S. patent application Ser. No. 952,660 discloses an arrangement of combination of the fuel assemblies which is effective in maintaining a substantially constant excess reactivity over the whole period of the operation of the nuclear reactor.
These prior arts, however, are measures for the reactor core in the state of equilibrium or the equilibrium core, and are not for the initial core, i.e. the core immediately after the building of the nuclear reactor.
Of course, a nuclear reactor operation with minimized control rod operation is possible even with an initial core, if the initial core is arranged such that fuel assemblies of different burning histories are combined with a new poison-containing fuel assembly, as taught by U.S.P. Appln. Ser. No. 952,660. However, it is difficult and impractical to charge a newly built nuclear reactor with fuel assemblies that have been used in another reactor or reactors.